AC Coupling in Utility-Interactive and Stand-Alone Applications

SMA recently released a US version of its Multicluster Box (MCB). The MCB connects up to four 3-phase three-inverter clusters for battery-based system capacities up to 110 kWac. Backup generators can...

While ac-coupling system architecture has been in development for about two decades and has been maturing in recent years, in some ways ac coupling is still the Wild West of renewable energy applications. The majority of the solar industry understands the design and installation of grid-direct inverter systems well. In ac-coupled applications, these grid-direct systems are integrated with battery-based inverter systems. The effective design and deployment of these combined systems quickly becomes complicated on several fronts.

First, ac coupling can be utilized in both utility-interactive and off-grid projects. These two system types have both fundamental and subtle differences in design requirements and operation when ac coupling is employed. Second, the majority of system designers and installers do not have much, if any, experience with battery-based systems, and even fewer have experience with ac-coupled systems. This results in a steep learning curve for many. Third, and perhaps most important, while fully integrated equipment platforms are available, many ac-coupled systems currently utilize equipment from different manufacturers. This mix of equipment adds complexity to ac-coupled system designs and presents challenges related to the amount of experience a given inverter manufacturer has with the use of its products in ac-coupled systems and the level of support it is able or willing to provide. For example, several grid-direct string inverter manufacturers, and most microinverter manufacturers, do not offer a warranty or provide support for their products if they are installed in ac-coupled systems.

An Introduction to AC Coupling

SMA Solar Technology began developing ac-coupling system platforms using its inverter products in the 1990s. I was first introduced to the concept while editing an article, “AC Mini-Grids,” for the October/November 2005 issue of Home Power magazine authored by Dana Brandt. The article reported on one of the first commercially deployed ac-coupled systems, which formed a microgrid in a remote village in Uganda. In the years that followed, SMA continued to develop and refine its products and integrated approach to ac coupling. In addition, inverter manufacturers Magnum Energy and Schneider Electric modified some of their battery-based inverter models for more seamless integration with ac-coupled systems by adding frequency-shift functionality to regulate battery charging. Currently, SMA and Schneider Electric manufacture both grid-direct and battery-based inverters that can be utilized to develop a complete ac-coupled system using equipment from a single manufacturer.

On a basic level, ac-coupled systems differ from their more conventional dc-coupled counterparts in one primary way—power generated by renewable resources, including PV arrays and wind or hydro turbines, is processed by grid-direct string inverters connected to the ac-output bus of a batterybased inverter system. Such systems can also integrate ac backup generators, with limitations in some cases, into the generation mix. In ac-coupled systems, grid-direct inverters essentially replace dc-charge controllers and frequently PV source-circuit combiner boxes as well. The absence of the dc-charge control in the system requires different and often combined approaches to battery-charge regulation. In ac-coupled systems, battery regulation is accomplished using several methods, including frequency-phase shift, blackout relays, and diversion controllers and loads that regulate the ac output of the grid-direct inverters. These approaches can range from seamless phase-shift approaches that modulate PV array generation based on ac-load demand and battery state of charge, to potentially complex networks of diversion loads driven by multiple voltagesense relays.

This article compiles information from several sources to present an industry-wide perspective on equipment selection and best practices for ac-coupling system design and installation in stand-alone and utilityinteractive systems. Four manufacturers of battery-based inverter/chargers provide technical insights related to the use of their products in ac-coupled system architectures. We also reached out to industry colleagues to share their direct experiences—both positive and negative—with deploying ac-coupled systems in the field. This issue of SolarPro also includes a comprehensive grid-direct inverter product specifications table (click here) that identifies which string inverter manufacturers offer a warranty on and support their products in ac-coupled applications.